A typical molten metal facility includes a furnace with one or more pumps for moving molten metal. During the processing of molten metals, such as aluminum, the molten metal is normally continuously circulated through the furnace by a centrifugal impeller pump, i.e., a circulation pump, to equalize the temperature of the molten bath. A typical furnace includes a pump well that is located between the heating chamber or hearth and the charge well (where raw material is inserted into the furnace). These three main sections of a typical furnace are fluidly interconnected with the circulation pump causing the molten metal to circulate from the pump well to the charge well to the hearth and back into the pump well.
In conventional direct-fired (fuel-fired) heating or melting furnaces, gas fueled burners produce a flame and/or products of combustion directly above the melt or load. Heat is transferred directly, from the flame and/or combustion products, to the melt by a combination of radiation and convection.
This method of melting has the problem that it is very inefficient. To prevent the waste gases/fuel from the burners coming into contact with the melt, the burners are typically mounted within the furnace at least four feet above the top surface of the molten metal (metal line). Because of this distance and although aluminum melts at just over 1200° F., conventional furnaces are run at approximately 2100° F. to ensure that a sufficient heat load is impinged on the metal to melt it fully (i.e., to the furnace floor). In this method of heating, large quantities of heat/energy are lost as they are exhausted up the stack.
Additionally, in these aluminum melting furnaces, the oxygen, hydrogen and carbon dioxide in the ambient air reacts with aluminum to form aluminum oxide or dross. Dross formation (i.e., aluminum oxidation) is undesirable in that it reduces aluminum yield. Depending on the type of charge materials to be melted, approximately 5% to 10% of the aluminum charged can be oxidized. This increases operational costs, due to the loss of the un-recovered aluminum, the labor and time requirements for skimming the dross from the furnace, and also energy losses from heating the dross within the furnace. That is, aluminum oxide has a characteristically low thermal conductivity and therefore greatly inhibits heat transfer to the molten aluminum as a dross layer acts as an insulator at the melt's surface, thus reducing the effectiveness of heat transfer from the burner to the aluminum.
For the above reasons, conventional furnaces operate at 20-30% efficiency because heat transfer to the melt in the furnace primary occurs through radiation from the overhead gas burners to the melt over a substantial gap and the insulative effect provided by the dross formed atop the molten metal.
There is therefore a need for a furnace that both reduces the thermal energy needed to effectively melt a heated product and significantly reduces the formation of dross in the molten metal.